In order to improve the characteristics of exhaust emissions and reduce combustion noise in engines, particularly diesel engines having a common-rail fuel injection system, a so-called multiple fuel injection pattern is adopted, according to which the fuel quantity to be injected in each cylinder at each engine cycle is split into a plurality of sub-injections. Thus, a typical multiple injection pattern may include a preliminary or pilot injection, which may be in turn split into two or more injection pulses, followed by a so-called main injection pulse, followed by a number of final injection pulses.
The pilot injection pulses have an effect both on the level of combustion noise and exhaust emissions, and their duration or energizing time (ET) is generally mapped in memories of the electronic injection control unit. The mapped values of the energizing time are predetermined with reference to an injection system having nominal characteristics (i.e., components having no drifts).
However, the fuel quantity which is actually injected by an injector into the corresponding engine cylinder is inevitably affected by drifts, with respect to the desired or nominal value and this, during the vehicle lifetime causes a variation of the combustion noise and exhaust emission characteristics.
Generally speaking, drifts of the injected fuel quantity may be caused by drifts of the injectors' characteristics, drifts of the rail-pressure sensors, drifts of the injector back-flow pressure, etc.
Among the above-mentioned causes of inaccuracy of the injected fuel quantity, the most critical one is represented by drifts of the injector characteristics. With the present injector manufacturing technologies, the fuel quantity actually injected in each cylinder for each engine cycle can be different from the desired (nominal) fuel quantity. The main reasons for this are the inevitable spread of the characteristics of the fuel injectors due to production tolerances and, subsequently, due to aging of the injection system.
As a result of production tolerances and their variations with aging, for a given energizing time at a given fuel-rail pressure, the actually injected fuel quantity can be different injector by injector.
The problem of a precise determination of the actually injected fuel quantity is particularly critical for small fuel quantities, for which a good precision and a high repeatability is necessary in order to achieve lower emissions and a reduced combustion noise.
US-2002/148441-A discloses a method for obtaining diagnostic information from instantaneous engine speed measurements and U.S. Pat. No. 6,021,758 A discloses a method for engine cylinder balancing using an engine speed signal.
As it will become apparent from the following description, the present invention is basically based on processing an engine speed signal, in order to achieve a reliable estimation of the quantity of fuel actually injected by each injector. Accordingly, it is desirable to improve the processing of an engine speed signal, in order to achieve a reliable estimation of the quantity of fuel actually injected by each injector. In addition, other desirable features and characteristics will become apparent from the subsequent detailed description, and the appended claims, taken in conjunction with the accompanying drawings and this background.